From the past, ceramic materials have been used for electronic equipment to absorb electromagnetic waves emitted therefrom or those penetrated therein. Particularly, in near field communication (NFC), a ceramic sintered body such as ferrite has recently been used as a magnet sheet for securing communication distance and reliability by suppressing an eddy current generated from a metal/conductive plate adjacent to an antenna and jamming radio waves caused by the eddy current. However, the ceramic sintered body is too brittle, and thus, if the ceramic sintered body has a small thickness, it is easily broken even by low pressure applied thereto. Therefore, it is not easy to apply the ceramic sintered body to a curved or flexible device requiring flexibility. Furthermore, since the ceramic sintered body has low flexibility, a process of attaching the ceramic sintered body to an NFC antenna is not easy. To solve such limitations, a method of providing flexibility has been known in which a blade cutter or laser facility is used to form a groove with an appropriate depth in a ceramic green sheet prepared through a tape casting method, then the green sheet with the formed groove is sintered to obtain a ceramic sintered sheet, and thereafter an adhesive layer and a protective layer are formed on or attached to the ceramic sintered sheet.
Japanese Patent No. 4277596 discloses a sintered ferrite substrate comprising an adhesive layer on one surface thereof, wherein the sintered ferrite substrate can be easily attached to or detached from a curved or uneven surface of an electronic device without irregular damage by forming one or more continuous U- or V-shaped grooves in at least one surface to allow the substrate to be separated along the groove. In such a sintered ferrite substrate, U-/V-shaped grooves induce regular cracks when a bending force is applied to the substrate, thereby preventing irregular cracks and also having flexibility together with the adhesive layer and the protective layer formed on each side of the substrate. However, high-priced equipment is required to form grooves, leading to an increase in production cost. Moreover, since the ferrite sheets are becoming thinner and slimmer in line with trends for making electronic products more lightweight, thin, and micronized, it is not easy to control the depth of the groove, which causes processing and management costs to increase and defects to increased. In addition, the tact time for the groove forming process is long, which decreases productivity and reduces the cost competitiveness of products.
Also, a conventional process may enable a ceramic sheet to have flexibility, but has problems in that flexibility is not maximally obtained and the ceramic sheet has poor flexibility in a certain direction in which the flexibility is not imparted, when a user desires to bend the ceramic sheet in that direction. Particularly, in an operation of providing flexibility to a ceramic sheet and performing a subsequent process, or attaching the ceramic sheet to an NFC antenna, the ceramic sheet is necessarily subjected to bending or impacts. At this time, the ceramic material properties of conventional ceramic sheets are deteriorated.